Any way to inspect multi-section can caps?

[MikeMcK]

My old BassMate has a 3-section Mallory can cap for filter caps... actually three 40 uF 450 VDC caps. Since I can't see them, is there any easy way to test them? Or should I assume that the fact it's an original Mallory assembly probably means they're too old to be in there?

The bigger issue is that the amp runs fine for awhile, then drops the volume. For you voracious readers, this is the same amp that blew a fuse the other day... turns out the last owner put a 1/2 amp fuse in there, and it's supposed to be 2A.

[SatelliteAmps]

Chances are the cap is probably failed or failing. Easy enough to replace it. Usually a volume drop off is a part failing somewhere in the amp. High voltage is a good place to look, and a failing cap could cause this. Then again, it could be a multitude of other causes.

[donnyjaguar]

I like to use the old caps myself. If they pass visual inspection (ie no leakage, dents or burn marks) I test them by removing them from the circuit and running high voltage via a 47k resistor into them. If they charge up fully and hold the charge they should be fine. The next test is to discharge them fully (snap!) and use my capacitance checker on my multimeter to confirm they are within specification.

[VaughnC]

If the caps are at least 15 years old they should be replaced whether or not they test or visually look good. It makes no sense to me to risk losing vintage transformer over a $20 filter cap.

[SatelliteAmps]

Especially on a gigging amp.

[mark norwine]

This tool rocks....I can't imagine not owning it.

http://www.midwestdevices.com/

http://www.midwestdevices.com/images/CwizWeb_AGE.jpg

[donnyjaguar]

In my experience, electrolytics generally die if left idle for years. If they test okay and are in an amplifier that is being used regularly you should be alright. I guess what I'm saying is I've never blown a power transformer this way. If the amp is fused properly this should protect you.

[SatelliteAmps]

I am curious, are you doing repairs on amps or is this just in a general on a few of your own amps? I don't like to trust old electrolytic caps in amplifiers that people want to keep running, or have some sort of value.

[donnyjaguar]

I design and build my own gear and also resurrect vintage musical instrument and hi-fi amplifiers, pre-amps and tuners. RF, analogue and power tech by training but also do digital. I also modify more modern tube hi-end gear. Fully competent in solid state also. I have on the bench right now a 1960's Philco monsterpiece from a friend. I won't say I've seen it all, but I've seen the good, bad and ugly. My repairs don't boomerang so I must be doing something right. I don't miss an opportunity to point out my pair of Eico HF20 (1957) are running on their original equipment electrolytics. That's 50 years so hence my thoughts on swapping out 15 year old 'lytics as a matter of course is the way it is.

[SatelliteAmps]

Thanks for the info. Nice to know a bit of backstory.